This invention relates generally to the atomization of fluids and more particularly to the atomization and entrainment of fluids, such as paint, for application to a surface.
Two types of apparatus for the atomization and particulation of fluids are currently in use, namely pneumatic and airless. As they refer to application of coatings both types comprise high pressure apparatus.
Airless equipment operate under ultra high pressures of 1800 to 2500 psig. Through the use of hydraulic pressure, the fluid is conveyed to a spraying apparatus where it is forced through a small orifice. The high pressure by which it is propelled causes it to pebble the surface finish. This high pressure is also responsible for a condition known as "bounceback" whereby the fluid literally bounces back into the atmosphere of the work place. It contaminates the environment, the worker and the equipment. It also wastes materials and requires considerable clean up time. Ultra high pressures make finish control impractical and can be dangerous to the user.
Conventional pneumatic apparatus utilize high pressure, low volume compressor or "shop air" in concert with an air regulator to atomize fluids. One of its chief features is it is inexpensive to operate and maintain, and is controllable. While the results are a fine finish, it has the disadvantage of producing a wasteful cloud of fluid and air commonly referred to as "overspray". The solvent vapors become part of the atmosphere and present a hazard not only to the environment but also to the atomizing equipment. The visible components of overspray are the solids which result in the degrading of the quality of the work surface and contamination of the work area. "Bounceback" of high pressure, airless systems also generate a type of overspray. This condition is created by the high velocity by which the fluid is propelled to the surface. The rebounding particles, solids and vapors, form an overspray cloud similar to that generated by pneumatic apparatus.
Overspray and its reduction has become a subject of major concern to any industry involved in the atomization of fluids. An amendment to the "Clear Air Act" of February, 1987 as it relates to hydrocarbon emission controls, established limitations and standards of performance for fluid transfer. Those industries affected are manufacturers and end users of commercial and consumer solvents, architectural coatings, pesticides, and all apparatus and methods involved in their application. Particular emphasis is being placed on government and military applicators. Additionally, individual states are implementing this act with their own pollution control bills. In some cases, such as California, high pressure paint systems and adaptations that rely on high pressure, low volume application of atomization are being studied for restricted use. This could have a disasterous effect on thousands of small businesses. Manufacturers of fluids, in order to reduce the percentage of carrier solvents, are now required to increase their solids content. This places a new burden on the atomizing system to atomize these high solids.
High solid materials are substantially more expensive and places new emphasis on transfer efficiency. An adaptation of the conventional method of spraying used where there has been a requirement for fine finishing and reduced overspray relies on the use of a high speed, electrically driven turbine. It delivers low pressure, high volume atomizing air and very successfully atomizes fluid viscosities of low to middle ranges. The apparatus has a transfer efficiency of approximately 70%. Transfer efficiency is expressed in percentages. For example, if a device or method is 70% efficient, it moves 70% of the solids of a container to the work surface. Conventional pneumatic systems have a common transfer efficiency of 35%. Airless systems are generally accepted to be 45% efficient.
Disadvantages of the turbine type equipment include its inability to transport low pressure air long distances. It does not have the power to successfully atomize high solid fluids, nor does it have the ability to adjust to varying viscosities. Because it is electric, it should not be used in explosive atmospheres such as aircraft hangers, ship interiors or inside paint booths.
All spraying systems require an apparatus to atomize the fluid and deliver it to the work surface. This apparatus is commonly called a spray gun. Guns vary in their configuration, size, weight and internal composition. Most attempts at improved fluid atomization for the purposes of spray painting have been directed to the gun. One such application centered on the location of a jet venturi induction pump located in the handle of the gun. Its failure to achieve commercial acceptability was due to the location to the jet venturi induction pump. Its function was to convert high pressure, low volume shop air to low pressure, high volume air. However, because of its close proximity to the work area, the ambient air drawn into the device was contaminated. Therefore the jet venturi induction pump was continuously introducing contaminated air into its internals. As a result, this contaminated air left deposits on the internal passages and orifices of the apparatus causing it to malfunction. Additionally, the user's hand could easily block the induction ports preventing a continuous inflow of ambient air. The entire system, as a result, was dominated by shop air. Lastly, the type of apparatus cannot be adjusted to meet varying fluid viscosities. All these adverse conditions negated the role of the devices as an improved method. To successfully atomize conventionally, pressures of 50 to 60 psig and 4 or 5 cfm. are required. The gun is designed to atomize fluids by the violent forward motion of the air as it exits the nozzle. Because the air nozzle is considerably larger than the fluid nozzle, it delivers more air than is necessary. The explosion into the atmosphere results in "overspray". There is a direct relationship between overspray and high pressure.
Therefore, there is an urgent need, according to standards now being introduced and currently in practice, for equipment that can meet these requirements. There is a need for apparatus that is capable of delivering low pressure, high volume air at acceptable transfer efficiency percentages, production rate standards and finish quality. Pressure control to atomize the new high solid fluids is also vital to its use. It must reduce volatile organic compounds to a level above new federal and state laws. The objectives of this improvement should be to reduce cleaning and maintenance problems. It should be portable and inexpensive to own. Furthermore, such apparatus should allow freedom of movement, ease of application and usable where paint booths are impractical or where atmospheres are explosive. The wide use of paint spraying booths has focused attention to such cost factors as filter maintenance and airflow rates. Additionally, there is a need for apparatus that can be used inside the usually explosive environment present in spraying booths. Ideally, this apparatus should be capable of using existing spraying devices such as pressure regulators, filters, air dryers, fluid mixing equipment, pressure vessels, and fluid lines. The savings, represented by such an improvement, would be incalculable to the fabricator and refurbisher in both the public and private sector.